Japanese Type100 HE Mortar Round with Type 100 selectable impact fuze,
used with the Type 97 (1937)
and later the Type 99 small trench mortar. This is based on the Stokes-Brandt
design as was the U.S. 81mm, and this round was actually interchangeable
with the U.S. M43A1 shell.
These had a maximum range of about 3,000 yards
Loaded weight: ~3kg (6.9lbs) of which 1 pound was TNT.

Evidently this was to make up for variations in manufacturing variables.
These weight indicators would be taken into account when aiming, to obtain
the desired ballistics.

The Type 100 Selectable Impact Fuze
can be set for instantaneous or delay function by turning a small adjustment
screw. Upon impact, the nose cap is forced in, breaking the shear wire safety
bringing the firing pin and primer together. The flash from the primer is
either allowed to directly enter the booster through the flash channel or,
depending on the orientation of the select screw, ignites a short delay train.

Diagram at right from TM9-1985-5
U.S. Army Technical Manual,
"Japanese Explosive Ordnance."
(A really great reference.)

Fuze Function Selector
Top: "Instantaneous" - Right: "Delay Action"

Osaka Army Arsenal 9; 8 10 Showa
(Sept 1943)

Mortar Increments
These are individual propellant packets which are removable. The
Type100 had six per round, but the actual number used depended on the flight
ballistics required for a particular shot, removing the unnecessary ones
prior to firing.

The bags are made of silk fabric and were
covered with some sort of waterproofing shellac, which evidently worked very
well as these were recovered from Guadalcanal in 1994 on the "Maruyama Trail", scattered in the rain forest
humus near a large pile 81mm mortar rounds. They have since been emptied of their contents, some sort of flaked nitrocellulose.
(Thank you Ewan & Steve!)

Translation
The character is iwa (also pronounced gan). It means rock, crag or reef.
There is a prefecture that starts with this character, Iwate. It is north
of the main island, and there is also a city by the same name. Apparently
there are no specialized military words than start with this character,
so a guess would be that this is a manufacturer's mark for the factory location,
Iwate. That would make the 20 (21) possibly a batch number. The 15.12 is most likely the date, December 1940.(Thank you Teri! - Teri's Nambu World )